Solid state lasers are generally known. Such devices are typically constructed by coupling a light-emitting diode to a resonant cavity.
A vertical cavity surface emitting laser (VCSEL) is one type of solid state laser. For example, 850 nm VCSELs may be built in the AlGaAs/GaAs material system and fabricated on a GaAs substrate. Like most semiconductor lasers, the active region of the VCSEL consists of multiple quantum wells, but, unlike edge-emitting lasers, the mirrors are formed during epitaxial growth using distributed Bragg reflectors (DBRs). The GaAs substrate functions to absorb photonic energies greater than the GaAs bandgap.
Most VCSEL devices are designed to emit light out of only one of the distributed Bragg reflector (DBR) facets. As such, associated transmission structures may be coupled directly to those facets.
While VCSEL lasers work well, they are still subject to failure and degradation due to time and temperature. Because of the importance of optical communications, a need exists for a means of monitoring VCSEL devices that is not subject to its own inherent defects.
VCSELs need some form of power control to maintain a constant output. Such power control could be performed automatically by measuring an output of a light emitting device during operation and using this measurement to control the power supplied to the light emitting device. Such control may be easily achieved when the light emitting device is an edge emitting laser because edge emitting lasers output light from two ends thereof. Thus, one output may be used for the desired application, while the other output may be used for the power control.
Previous attempts to monitor the power of VCSELS typically involve splitting off a portion of the output beam to use as a monitor beam. However, such splitting off obscures part of the beam which may affect the wavefront and imaging, and hence coupling, of the light. Further, if the intensity distribution changes, such as when there is a change in lasing mode, the monitored power may change in a way which does not represent the overall output power of the VCSEL within a desired lasing mode.
Additionally, splitting off a portion of the beam may require that the output of the VCSEL to be increased in order to maintain the requisite power level at a laser receiver while allowing the monitoring function. Previous methods of scattering the beam to create a monitor beam relied on reflection for directing the beam and did not provide an optimal signal to the monitor detector. Further, previous scattering did not insure the entire beam was being monitored. Beam splitting may also require complex optical reflecting components that can be costly and involve precise alignment steps.
In this invention is disclosed a novel method for monitoring the output performance of a VCSEL array of optical ports. The invention uses light from the VCSEL that is divergent from the optical signal entering the optical waveguide. In other words, light not entering the waveguide because of natural optical losses is thus utilized for the purpose of monitoring the VCSEL. By using light that would otherwise be scattered or absorbed (lost light), more light is transmitted down an optical fiber, and signal integrity may be preserved.